Impacts of Heat Treatment and Long-Term Storage on Methylaluminoxane Composition, Catalytic Activity, and Polyethylene Properties

Abstract

Methylaluminoxane (MAO) is a critical activator in olefin polymerization, but it exhibits limited stability under high temperatures or prolonged storage. In industrial processes, MAO is typically concentrated by heating in toluene under vacuum and stored for months before use, however, the effects of these processes on its stability are poorly understood. We investigate the stability of freshly prepared MAO subjected to heat treatment and long-term storage, focusing on changes in aluminum content, methyl-to-aluminum ratio, polymerization activity, and properties of the resulting polymer. Our results demonstrate that higher temperatures, extended treatment or storage times, and increased initial MAO mass fractions contribute to gel formation within MAO solutions, declining cocatalytic activity. Using heat-treated-MAO/Cp₂ZrCl₂ for ethylene polymerization results in PE with reduced molecular weight compared to fresh MAO, while the effect on dispersity (D̵) is complex, exhibiting both increases and decreases in different cases. The aging of MAO leads to the formation of insoluble, high-molecular-weight gels. Analysis of the gel phase reveals an elevated Me/Al ratio, indicating selective partitioning of MAO species during gelation. Scanning electron microscopy (SEM) reveals that these gels consist of coalesced, interconnected clusters with a porous, amorphous morphology composed of nanoscale primary particles. Such morphological evolution correlates with the observed decline in polymerization activity. Our findings offer insights for optimizing MAO concentration processes and storage conditions to preserve its performance in industrial applications.